Changes in the risk of schizophrenia in Scotland: is there an environmental factor?

Various studies have suggested that there may be an environmental factor in schizophrenia acting before or at birth but with delayed effects. Evidence that the risk of developing schizophrenia varies randomly with the year of birth would help confirm the existence of such a factor. Data from the Scottish Health Service Information and Statistics Division, comprising all first admissions for ICD-9 schizophrenia in the years 1963-91, were used to estimate the lifetime risk of developing schizophrenia for each year of birth from 1900 to 1969. In the period after 1928 the lifetime risk steadily declined. The rate of decline was greater in females. The random fluctuations in the risks in females did not change systematically, but there was a significant decline in the variability of these fluctuations in males. These random fluctuations suggest a possible role for randomly varying environmental influences around the time of birth. Our findings suggest a possible diminution in the role of such environmental influences in schizophrenia among males in Scotland in the years 1929 to 1969.